1. Field of the Invention
The present invention relates to a method and apparatus for connecting a well on the ocean floor with a wellhead "Christmas" tree, (i.e., the flow control valve assembly) on a fixed or relatively fixed platform, such as a floating tension leg platform, or the like. More particularly, the present invention relates to a riser top joint used in completing such a connection that makes it unnecessary to precisely measure the distance between the well and the wellhead valve assembly. Even more particularly, this invention relates to a threadform design that will withstand high longitudinal stresses to which the riser top joint is subjected.
2. Description of the Related Art
In the exploration and drilling for offshore oil and gas wells, one form of marine structure found to be desirable and effective is the tension leg platform. In this type unit, the working structure is floatably supported by its own buoyancy. However, tension cables applied to the lower end of the platform and fixed to the ocean floor, allow it to be drawn downwardly to a desired working depth to improve stability.
In the drilling of offshore wells, it is necessary to utilize a riser, often referred to as a marine riser, which extends from the well head to the working deck of the floating platform. The riser member is in effect an elongated enclosure which surrounds and protects the drill string, production and/or injection tubing, the oil or gas export riser, as well as pipes which pass from the well upwardly to the platform deck.
Such risers are necessary for normal drilling and/or production operations but are susceptible to damage and in many cases to breakage. The latter results from excessive strain stress to the riser as the floating platform vacillates about its working position in response to excessive wind and wave conditions at the water's surface.
Further, the riser is subjected to a considerable stress induced by water currents and the like which pass around the riser, but which are not particularly effective against the platform. In such an instance the normally vertical riser disposition tends to be distorted as the latter is displaced laterally in one or more directions in response to underwater currents. One of the benefits of a tension leg platform over other floating systems is the very small vertical oscillation that occurs. The structure is less susceptible to natural forces such as wind and waves which would otherwise tend to displace and disturb the horizontal orientation of the platform with respect to the ocean floor. This enables the wellhead valve assembly to be mounted within a few feet of a platform deck without the need for some complex form of motion compensation system. However, the use of a rigid riser system requires a precise measurement between the well on the ocean floor and the deck of the platform, in order to obtain a riser of the necessary length. Such precise measurement becomes increasingly difficult as the water depth moves from hundreds to thousands of feet deep.
The invention disclosed in U.S. Pat. No. 4,733,991 issued Mar. 29, 1988 to Myers is an attempt to alleviate some of the prior art deficiencies. Myers discloses the use of a first series of protrusions on the marine riser top joint to provide a plurality of connecting points for the wellhead tree, and a second series of protrusions on the marine riser top joint to provide a plurality of connecting points for the riser tensioner means.
In the known prior art designs, the protrusions are generally "V" shaped and are sometimes referred to as V-threads. A V-thread protrusion on one piece will fit into the corresponding thread root between two V-thread protrusions on the piece to which it is mated. FIG. 1 is a representative illustration of a marine riser 11 with V-thread protrusions shown generally at 12, mated with a riser tensioner ring upper slip 14 having V-thread protrusions shown generally at 15. FIG. 2 is an enlarged view of V-thread protrusions 14 of upper slip 14. V-thread protrusion 12a of marine riser 11 fits into the corresponding thread root 15b, between two V-thread protrusions 15a of upper slip 14. Likewise, V-thread protrusion 15a of upper slip 14 fits into the corresponding thread root 12b, between two V-thread protrusions 12a of marine riser 11.
In the normal course of operation, the platform is subjected to wave, wind and other forces that impart a high longitudinal force on the riser tensioner connections. Unfortunately, these forces to which the platform is subjected to, can force platform members such as the riser tensioner slip and the marine riser together. If the members are mated together with a V-thread or similar design, the V-thread protrusions from one member act as a wedge in the corresponding thread root between two V-thread protrusions on the mated member, resulting in a high tensile stress area prone to stress cracking. For example, in FIG. 1, as marine riser tensioner lower slip or tensioner ring 18 is forced in the direction of direction arrow 3, the angled saw cut between upper slip 14 and tensioner ring 18, will force upper slip 14 toward marine riser top joint 11. As the V-thread protrusions on marine riser top joint 11 and upper slip 14 are forced into their corresponding thread roots, high tensile stress areas on the marine riser top joint 11 and upper slip 14 develop, referred to generally in FIG. 2 as 12c and 15c respectively. These areas are prone to developing stress cracks such as stress crack 5.
Generally these high tensile stress areas may be reinforced by making the marine riser top joint 11 and the upper slip 14 thicker at these points. However, this solution tends to increase the size and weight of the member. While such reinforcement is adequate for the upper slip, design considerations limit the degree to which the marine riser may be reinforced. With the marine riser, weight considerations are an important factor. Also, since many pieces of equipment or machinery are sized off of the diameter of the marine riser, it is crucial that the marine riser conform to industry diameter standards. It would be beneficial to provide a thread design which could be utilized in the marine riser that would reduce the stress in the thread roots, and improve the fatigue life of the riser while also minimizing the wall thickness and weight of the marine riser.
Therefore, a need exists in the industry for a lightweight connector for connecting the riser tensioner and wellhead valve assembly to the marine riser that can more adequately withstand the rigors of the longitudinal forces that are generated in the normal course of operation.